There are many industrial applications and processes that require precise patterning of a workpiece, two such applications being, for example, fabricating microcircuits, and forming circuit board interconnections. For instance, the demand for compact electronics packaging has seen the means for forming interconnections among microcircuits evolve from the use of peripheral interconnections (i.e., connections around the edge of the package) to the use of flexible ball grid arrays (BGA) on the surface of the package. This newer BGA packaging and thin, flexible interconnection method requires the creation of an array of hundreds of vias (i.e., holes) on the order of 25 μm diameter in a thin multilayer laminate insulating layer, such as polyimide (for example, KAPTON polyimide, sold under this trademark by DuPont).
Traditional means for accomplishing precise patterning of a workpiece by micromachining include mechanical drilling, chemical etching, contact printing, and projection photolithography. In recent years, however, lasers have been shown to be a valuable and often preferred means for performing high-precision micromachining because of their directionality, coherence, high intensity and high photon energy.
The specific interaction between the laser beam and the workpiece depends on the laser wavelength and the material comprising the workpiece. For instance, infra-red wavelength and visible wavelength laser beams focused to a small spot on the workpiece provide intense localized heating which vaporizes most workpiece materials. However, such localized heating can have the undesirable side-effect of thermally damaging the workpiece. On the other hand, ultraviolet (UV) wavelength lasers (such as excimer lasers) provide photons with sufficient energy to excite the electrons that form the molecular bonds of certain workpiece materials such as polyimide. Sufficient excitation of the bonding electrons with a tightly focused beam results in the localized disassociation of the material with little or no heating of the workpiece. This process is referred to as “ablation.”
In a typical laser-based micromachining application, a laser is used to irradiate the surface of a workpiece in order to form a desired pattern thereon or therein. One method of laser-based micromachining involves a mask-based step-and-repeat operation, wherein the mask is illuminated with a laser beam, and a projection lens images the mask onto the workpiece. While this method is capable of forming small well-defined spots and is well-suited for forming arbitrary shapes or figures, the method is inefficient with its use of available light because the mask blocks a portion of the beam in order to form the pattern. Also, the step-and-repeat method is time-consuming, particularly when hundreds or thousands of spots need to be patterned on each of a multitude of workpieces.
Another method of laser-based micromachining involves scanning a laser beam over the workpiece with a flying-spot scanning apparatus. However, this apparatus is fairly complex and expensive, and is generally not well-suited for forming arbitrary shapes and figures, and it has limited processing capacity or “thruput” (up to about 1000 holes/second) because of its serial mechanical nature.
To increase “thruput” (the number of workpieces that can be processed in a given time interval) and to simplify the apparatus for step-and-repeat laser micromachining, there have been recent efforts to develop laser micromachining methods and apparatus that employ various types of multiple-focusing means for simultaneous drilling multiple holes (i.e., forming holes in “parallel” rather than serially). Such means include conventional lenses, fresnel zone plates (FZP's), computer-generated holograms (CGHs), diffractive optical elements, and binary phase gratings.
Because there is some confusion in the patent literature regarding the definition of the above multiple-focusing means, the following definitions are used herein.
A FZP is a plate with concentric transparent and opaque annular rings or ring sections that transmit and block alternating Fresnel zones on a wavefront thereby allowing the transmitted light to positively interfere and come to a focus. An FZP can also be made with refractive zones instead of opaque zones, so that the phase of the light is changed to be in phase with the other zones, rather than simply being blocked. For FZP's used to create an image other than a single focus spot, the zone pattern is calculated and then produced by digital means and lithography, as is referred to as a “kinoform.”
A holographic optical element (HOE) is an optical component used to modify light rays by diffraction, and is produced by recording an interference pattern of two laser beams and can be used in place of lenses or prisms where diffraction rather than refraction is desired.
A hologram is a continuous diffracting region created by two or more interfering beams in which the phase information of the wavefronts in the object is converted to intensity or phase variations. The continuous diffracting region can also be computer-generated. Each point on the hologram contains information about the entire object, and thus any portion of the hologram can, in principle, reproduce the entire three-dimensional image of the object via wavefront reconstruction.
Diffractive optical elements (DOEs) have zones of refraction, phase shift, or amplitude modulation with a scale that allows for the directional control of diffraction effects. A DOE can have a focusing effect as in an FZP, or it can have more complicated effects such as chromatic correction or aspherical distortion correction. Diffracting optical elements are made using computation to describe the zones of diffraction, and then producing these zones in a suitable substrate surface by means of diamond turning or by lithographic processes common to semiconductor manufacturing or injection molding.
A binary optical element is a diffracting optical element having a binary or “flat-top” zone profile.
In addition, the phrase “in-line” as used herein denotes a geometry in which is coaxial, i.e., disposed along a common axis.
Laser micromachining methods and apparatus employing the above multiple-focusing means are generally faster and more efficient than step-and-repeat micromachining, contact printing, and projection photolithography. However, these multiple-focusing apparatus and methods also have their own shortcomings and limitations.
U.S. Pat. No. 5,233,693 to Zumoto et al. discloses an in-line optical projection micromachining apparatus. The apparatus comprises a mask having apertures and reflective parts in between, and a hemispherical reflective member for returning the light reflected off the reflective parts of the mask back toward the open areas of the mask. A projection lens is used to image the mask onto a workpiece. While this system operates in-line, it is fairly complex because the projection lens for most applications would not be a single lens element, but a multi-element well-corrected lens system capable of imaging very small features. In addition, when the mask features to be patterned are small relative to the total area of the mask, the amount of light transmitted by the mask will be relatively low, even with the hemispherical reflective member present.
U.S. Pat. No. 5,481,407 (the '407 patent) to MacDonald et al. discloses a laser-based method and apparatus for creating small holes having a desired shape (e.g., circular, square, oval, etc.) by laser ablation. The focusing means is a segmented array of FZPs, wherein the form of the individual FZPs comprising the segmented array determines the shape of the holes. While this technique allows for a multitude of holes to be patterned simultaneously with a single exposure, it is not well-suited for patterning generalized “non-hole” type objects, i.e., objects having significant physical extent. This is because each FZP in the FZP segmented array is designed to bring light to a small focus at a designated location on the workpiece, rather than to form an image of an extended object on the workpiece. This is disadvantageous because each of the multitude of discrete FZP elements needs to be aligned to a specific location on the workpiece.
Moreover, there are practical shortcomings with the focusing means disclosed in the '407 patent. For instance, the image-forming properties of a segmented lens array are disadvantageous in an industrial environment. Generally, when a workpiece is patterned with a laser micromachining apparatus, material on the workpiece is ejected from the surface during patterning and can become deposited on the image-forming means of the apparatus. When the image-forming means is a lens-type array (e.g., an FZP array), the deposited material can obscure a portion of the array, resulting in a diminution of image quality in the patterns formed by the obscured array lens elements. This problem can be particularly troublesome when the ablated material is transparent, because the deposited material will create a phase error over portions of the lens-type array which is difficult to detect by visual inspection. To prevent ejected material from depositing on the image-forming means, a pellicle or other protective surface can be introduced into the apparatus. However, such modifications make the apparatus more complex and costly.
Also, because the FZPs are discrete, it is difficult to make a mask that will print is structures in close proximity.
The publication “Laser Machining with a Holographic Lens,” Applied Optics, Vol. 10, No. 2, February 1971 by J. M. Moran discloses the concept of using a hologram illuminated by a laser for machining single and multiple spots on a workpiece. Using a hologram as a mask is advantageous in that it can have multiple-focal distances because of the three-dimensional nature of the holographic image. A hologram also has the advantage of being able to create not only sharply focused points, but extended images which can be patterned into or onto a workpiece. Moreover, there is no need to compute an “array” of segmented areas to achieve repetitive patterning, as a hologram can comprise a substantially continuous diffracting region recording of the wavefronts from disparate features on an object. In other words, a hologram is not a segmented array. Rather, each portion (or, alternatively, large portions) of the hologram contributes to the creation of the image formed. Indeed, a hologram can be cut into pieces, with each piece being capable of reproducing, in toto, the entire image (albeit from a limited set of angles). This property makes holograms very advantageous over discrete arrays of focusing elements because if part of the hologram is obscured by for example material ejected from the workpiece, the first-order net effect of the obscuration is a diminution in the overall intensity of the entire image, rather than the loss of resolution of the individual sub-images.
While the hologram in the above-cited publication has the above-mentioned advantages, it is used off-axis, meaning that the illuminating beam, hologram, and workpiece are not in-line. An in-line geometry is preferred for most manufacturing applications, as the apparatus is simpler to fabricate and less costly than an off-axis apparatus. Also, the method of patterning with an in-line apparatus is less complex, as precise alignment between the workpiece and the hologram is more easily achieved. Moreover, an in-line geometry allows for the hologram to be “replayed” with a beam having a wavelength different from the wavelength used in its construction with minimal impact on aberrations. In addition, some manufacturing processes require an in-line geometry because of the geometry of the existing installed base of expensive manufacturing apparatus. Also, for many applications, e.g., drilling vias for microcircuit interconnections, the vias must have an axis perpendicular to the surface of the workpiece in order for the various layers of the microcircuit to be properly interconnected.
U.S. Pat. No. 5,612,986 to Howells et at. (the '986 patent) discloses a method of performing X-ray lithography using holographic images from a computer-generated on-axis hologram. However, the method disclosed in the '986 patent requires a computer-generated hologram (which restricts the types of images the hologram can form and is computationally intensive), is only for forming images smaller than 0.25 μm, and apparently only works at X-ray wavelengths.
U.S. Pat. No. 4,668,080 to Gale et al. (the '080 patent) discloses an apparatus for forming a periodic pattern in a layer of photosensitive material, the apparatus comprising a lenticular array of lenslets and a means for scanning a beam of light sequentially through each lenslet in the array. The '080 patent also discloses an apparatus where the lenslets in the lenticular array are holograms, and where the array of holograms is sequentially scanned by a light beam scanning means.
The publication “High-resolution image projection at visible and ultraviolet wavelengths,” by I. N. Ross et al., Applied Optics, Vol. 27, No. 5, pg. 967 (Mar. 1, 1988) discusses the construction of a holographic test mask having resolution test-patterns recorded therein, and then patterning the test-patterns in photoresist by illuminating the holographic test mask with a laser. While this technique exploits the aforementioned advantages of a hologram, the recording of the hologram and subsequent patterning steps are accomplished off-axis.
The publication “Photosensitized polystyrene as a high-efficiency relief hologram medium,” by F. M. Schellenberg et al., SPIE Vol. 1051 Practical Holography III (1989), discloses using holograms off-axis for photoablation using high-powered lasers. The holograms were reflection holograms formed in t-BOC, a plastic material with limited damage threshold to deep ultra-violet wavelengths.
The publication “A technique for projection x-ray lithography using computer-generated holograms” by C. Jacobsen and M. R. Howells, J. App. Phys. 71 (6) 15 March 1992, discusses a holographic approach to x-ray projection lithography using an in-line hologram generated by computer. However, this publication only provides computer simulations of the imaging and contemplates an in-line CGH, which is time-consuming. Indeed, while the theoretical aspects of in-line holograms have been explored, the actual fabrication of in-line holographic masks for practical industrial use is truly daunting. The fact that persons skilled in the art of holography have not, to date, actually optically fabricated and used an in-line holographic mask suitable for micromachining in an industrial environment is testimony to the difficulty involved in applying in-line holographic methods to an industrial environment.
Therefore, there exists a need for high-efficiency optically fabricated in-line holograms suitable for industrial use from the infra-red to the deep ultra-violet region of the electromagnetic radiation spectrum for patterning a workpiece.